Add language declarations to code blocks

Adding code language declarations to the last round of files in
the API folder.

doc/api/version.md

@@ -5,7 +5,7 @@
 Retrieve version information for this GitLab instance. Responds `200 OK` for
 authenticated users.
 
-```
+```plaintext
 GET /version
 ```
 

doc/api/visual_review_discussions.md

@@ -10,7 +10,7 @@ feedback from [Visual Reviews](../ci/review_apps/index.md#visual-reviews-starter
 Creates a new thread to a single project merge request. This is similar to creating
 a note but other comments (replies) can be added to it later.
 
-```
+```plaintext
 POST /projects/:id/merge_requests/:merge_request_iid/visual_review_discussions
 ```
 

doc/api/vulnerability_findings.md

@@ -34,7 +34,7 @@ Read more on [pagination](README.md#pagination).
 
 List all of a project's vulnerability findings.
 
-```
+```plaintext
 GET /projects/:id/vulnerability_findings
 GET /projects/:id/vulnerability_findings?report_type=sast
 GET /projects/:id/vulnerability_findings?report_type=container_scanning

doc/api/wikis.md

@@ -8,7 +8,7 @@ Available only in APIv4.
 
 Get all wiki pages for a given project.
 
-```
+```plaintext
 GET /projects/:id/wikis
 ```
 
@@ -49,7 +49,7 @@ Example response:
 
 Get a wiki page for a given project.
 
-```
+```plaintext
 GET /projects/:id/wikis/:slug
 ```
 
@@ -77,7 +77,7 @@ Example response:
 
 Creates a new wiki page for the given repository with the given title, slug, and content.
 
-```
+```plaintext
 POST /projects/:id/wikis
 ```
 
@@ -107,7 +107,7 @@ Example response:
 
 Updates an existing wiki page. At least one parameter is required to update the wiki page.
 
-```
+```plaintext
 PUT /projects/:id/wikis/:slug
 ```
 
@@ -138,7 +138,7 @@ Example response:
 
 Deletes a wiki page with a given slug.
 
-```
+```plaintext
 DELETE /projects/:id/wikis/:slug
 ```
 
@@ -160,7 +160,7 @@ On success the HTTP status code is `204` and no JSON response is expected.
 Uploads a file to the attachment folder inside the wiki's repository. The
  attachment folder is the `uploads` folder.
 
-```
+```plaintext
 POST /projects/:id/wikis/attachments
 ```